Up to now, a technology for estimating stiffness of a blood vessel or a blood pressure from a pulse wave signal or an acceleration pulse wave signal that is obtained by differentiating the pulse wave signal has been known. In order to estimate the stiffness of the blood vessel and the blood pressure correctly, a noise contained in the pulse wave signal or in the acceleration pulse wave signal is necessary to be reduced. Patent Document 1 (JP 3965435 B2) has proposed a method for reducing the noise contained in the acceleration pulse wave signal. First, in this method, the acceleration pulse wave signal is partitioned into multiple sections corresponding to respective pulses of a pulse wave. Next, the multiple sections are arithmetically averaged such that base points of the respective sections become coincident with each other. Lastly, the stiffness of the blood vessel or the blood pressure is estimated from the arithmetically-averaged acceleration pulse wave signal.
In the method of Patent Document 1, a wave profile of the arithmetically averaged acceleration pulse wave signal may become broad. Thus, necessary information for the estimation of the stiffness of the blood vessel or the blood pressure may be lost. This may be because the arithmetical averaging has not been performed correctly since the base point in each section is incorrect.